The present invention provides a novel method for recovering and reprocessing superabsorbent polymer fines. The method provides for the reconstitution of the fines back to superabsorbent polymers useful in applications, such as, diapers, sanitary napkins, soil additives, fuel filters, desiccants, cements, and sludge dewatering.
Hydrogel-forming polymer compositions (hereinafter referred to as superabsorbent polymers) are used as absorbents in absorbent structures and absorbent articles. Synthetic superabsorbent polymers are typically produced by one of the following methods: gel, inverse suspension, or modified bulk polymerization. Typical gel methods for producing superabsorbent polymers are described in U.S. Pat. Nos. 4,654,039 (Brandt et al.) issued Mar. 31, 1987, and 4,286,082 (Tsubakimoto et al.) issued Aug. 25, 1981. Typical inverse suspension processes for producing superabsorbent polymers are disclosed in U.S. Pat. Nos. 4,698,404 (Cramm et al.) issued Oct. 6, 1987, 4,446,261 (Yamasaki et al.) issued May 11, 1984, and 4,340,706 (Obayashi et al.) issued July 20, 1982. A modified bulk process is described in U.S. Pat. No. 4,703,067 (Mikita et al.) issued Oct. 27, 1987.
The Brandt et al. patent discloses a gel process comprising the steps of preparing a reaction mixture consisting essentially of particular amounts of unsaturated polymerizable acid group-containing monomers, crosslinking agent and optionally free radical initiator in an aqueous medium; subjecting this reaction mixture to polymerization conditions to produce a substantially water-insoluble, slightly crosslinked polymer material having under certain conditions particular gel volume, gel strength and extractable polymer content characteristics; and neutralizing at least a portion of the acid functional groups of the resulting polymer material with salt cations to form a partially neutralized polymer material having a degree of neutralization of at least 25%. The hydrogel material formed in accordance with the Brandt et al. patent may optionally be dried in order to prepare absorbent hydrogel-forming polymer materials which reform hydrogels upon subsequent contact with water or body fluids.
A typical inverse suspension process for producing superabsorbent polymers is disclosed in the Cramm et al. patent. This process provides for the suspending of an aqueous solution of acrylic acid and an alkali metal acrylate or ammonium acrylate, the mole ratio of the acrylic acid to the alkali metal acrylate or the ammonium acrylate being 50/50 to 2/98, in an alicyclic or aliphatic hydrocarbon solvent containing a surfactant having an HLB value of 8-12; subjecting the resulting suspension to inverse suspension polymerization in the presence of a water-soluble radical polymerization initiator and; if necessary, crosslinking the resulting polymer with a crosslinking agent, whereby the polymerization is conducted in the presence of a water-soluble chain transfer agent. The water-soluble chain transfer agent being formic acid.
Mikita et al. disclose a modified bulk process for producing superabsorbent polymers comprising the steps of combining potassium acrylate and a polyvinyl monomer with water in an amount of 55 to 80 combined weight percent of potassium acrylate and polyvinyl monomer based on the total weight of potassium acrylate, polyvinyl monomer and water to form a monomer mixture; and adding a polymerization initiator to the monomer mixture to initiate polymerization of the monomer mixture. The monomer mixture is polymerized without external heating by utilization of exothermic heat of reaction as substantially the only non-ambient energy source to drive water away from the polyacrylate resin to form the crosslinked polyacrylate resin having a water content sufficiently low, i.e., 15% or less, to be powdered, such as by pulverization, without an intermediate drying step.
Each of the aforementioned processes used to produce superabsorbent polymers may generate superabsorbent polymer fines. Superabsorbent polymer fines are defined as those particles which are too small in size for current market applications, e.g., diapers, sanitary napkins, etc. The fines may be generated either during the polymerization step of the process or during subsequent processing steps when the superabsorbent polymer is ground and sieved to meet desired particle size specifications.
As an example, in the production of superabsorbent polymer material for use as an absorbent in a certain personal product application all particles smaller than about +280 mesh are considered fines. Fines recovered from superabsorbent material used in this particular application may amount to between 5-35% of the total polymer produced.
Since superabsorbent fines are those particles which are too small in size for a specific market application, they are typically removed from a commercial product via a size classification step. The resulting fines are typically stored or disposed of until a suitable market can be found for them. Commercial applications of superabsorbent fines are limited by the small particle size which typically results in handling problems, such as, gel blocking and dusting.
Retaining fines in commercially used superabsorbent polymer material may also result in gel blocking, i.e., the fines block larger particles causing reduced absorbent capability of the total material. Due to gel blocking effects, fines are normally separated from the superabsorbent polymer material prior to commercial application.
Due to the high disposal, storage and handling costs associated with superabsorbent polymer fines, it is desirable to develop new methods for handling and utilizing these fines. The present inventors have discovered a novel process for recovering and reprocessing superabsorbent polymer fines making them suitable for commercial applications.
Others have attempted to convert polymer fines to commercially acceptable polymer material by agglomerating the fines together by either heating or addition of a chemical binder. One such agglomerating technique is disclosed in U.S. Pat. No. 3,514,510 (A. Hoffman) issued May 26, 1970. The Hoffman patent discloses a process wherein polymer fines are agglomerated in a fluidized bed maintained in a fluidized state with a mixture of two gases, the minor portion of which was heated above the polymer softening point, to cause agglomeration in a localized area. The polymer powder was agglomerated in a local area and the heavy agglomerate settled to the bottom of the fluidized bed and was withdrawn while fresh powder was added to the softening zone.
The problem with agglomerating polymer fines by heating or chemical binding is that attrition of the agglomerated particle may occur during commercial production or use. The severed fines are generally undesirable since they are typically smaller than the acceptable particle size range and may cause gel blocking and handling problems as described above.
Thus, the present invention provides a unique process which overcomes the disposal and handling problems associated with superabsorbent polymer fines, as well as avoids the attrition problems typically associated with agglomerated particles. The advantage of the present method is that the superabsorbent polymer fines are recovered and reconstituted into a commercially acceptable amorphous superabsorbent polymer material. Furthermore, the process of the present invention avoids the high cost and dust handling problems associated with storage, disposal and use of the fines. Additional advantages of the present invention shall become apparent as described below.